You joke but make a good point, most major installations have backup diesel powered generators to provide power for a certain limited but generous time.

What happened with Heathrows generators? Did they kick in? Do they have any?

Edits: yes they did kick in as normal https://www.standard.co.uk/news/uk/heathrow-ed-miliband-nati...

Note that usually backup generators are only for essentials like air traffic control, landing lights, and operating emergency stuff.